1. Field of the Invention
The present invention relates to a method and apparatus for grinding wafers using a grind chuck having a high elastic modulus. The present invention relates more particularly to an apparatus and method for grinding wafers capable of protecting semiconductor chips formed on the front face of the wafer, without using an ultraviolet tape attached thereto, while protecting the semiconductor chips from being contaminated and damaged.
2. Description of the Related Art
Generally, thin semiconductor packages are produced by forming semiconductor chips on the front surface of a wafer and grinding the rear face of the wafer by means of a grinder.
A conventional wafer grinding machine includes a process chamber which provides space for performing the grinding operation. A grind table is installed partially inside and partially outside of the process chamber. A plurality of grind chucks for holding a wafer by suction force are mounted on the grind table. A first grind unit installed in the process chamber first grinds the rear face of the wafer held by the grind chucks until the wafer has a predetermined thickness. A second grind unit grinds the first-ground rear face of the wafer again until the wafer has a desirable thickness. An intake and exhaust groove is formed through the grind table to establish a vacuum pressure to hold the wafer on the grind chuck and to exhaust air therethrough so as to release the wafer from the grind chuck. A deionized water supply duct is also employed in the wafer grinding machine to remove heat generated while the wafer is ground by spraying deionized water onto the wafer.
A porous portion filled with a plurality of pores is formed in the central region of the grind chuck. The porous portion corresponds to the intake and exhaust groove so that the wafer can be held by or detached from the grind chuck by suctioning or exhausting air. The grind chuck is formed of ceramic material having a high hardness. An upper surface of the grind chuck is periodically reground so that the flatness of the grind chuck can be consistently maintained at a fixed level.
Conventionally, an ultraviolet tape is attached to the front face of the wafer where the semiconductor chips are formed. When the rear face of the wafer is ground, the ultraviolet tape and the flow of deionized water prevent silicon dust from contacting the front face, and thereby protect the semiconductor chips formed on the front face of the wafer from being contaminated by the silicon dust. Furthermore, the ultraviolet tape has a cushioning effect which protects the semiconductor chips from stress caused by the grinding operation.
After the ultraviolet tape is attached to the front face of the wafer, the wafer is loaded on the grind chuck in such a manner that the surface of the grind chuck is in contact with the front face of the wafer. Then, a vacuum pressure is generated in the intake and exhaust groove. As a result, the wafer is held on the grind chuck.
Thereafter, the grind chuck with the wafer held thereon is moved to the first grind unit inside the process chamber by rotating the grind table. Deionized water is then applied to the wafer. While rotating at a fixed velocity, the first grind unit moves downwardly to the wafer whereby the rear face of the wafer is ground to a predetermined thickness. Since the first grind unit rotates slowly, the first grinding operation leaves a rough rear face on the wafer.
The grind table is rotated again, and the first-ground wafer is transferred to a location beneath the second grind unit. Then, while rotating at a higher velocity than the first grind unit, the second grind unit moves downwardly to the transferred wafer in the same manner as the first grinding operation. As a result, the rear face of the wafer is secondly ground to a desirable thickness. Since the second grind unit rotates at a higher velocity than the first grind unit, the second grinding operation leaves the rear face of the wafer smooth.
When the wafer is ground to the desirable thickness through the first and the second grinding operations, the wafer is unloaded from the grind chuck. The front face of the wafer is then exposed to ultraviolet ray to reduce the adhesion of the ultraviolet tape attached to the front face of the wafer. Thereafter, a removing tape having a higher adhesion than the ultraviolet tape is attached to the ultraviolet tape to remove the ultraviolet tape from the front face of the wafer.
However, such a grinding process carried out after an ultraviolet tape is attached to the front face of the wafer suffers several problems. First, since the ultraviolet tape is expensive, the manufacturing costs of the products increase. Second, the ultraviolet tape attached to the front face of the wafer incurs cumbersome additional steps of exposing the wafer to ultraviolet rays, attaching a removing tape to the ultraviolet tape, and removing the ultraviolet tape using the removing tape. This results in a complicated grinding process, increased operation time and reduced productivity.
Third, the air between the front face of the wafer and the adhesive tape is repeatedly contracted and expanded by the heat generated while the rear face of the wafer is ground and the deionized water is sprayed to remove the heat generated by the grinding. As a result, a gap is generated between the front face of the wafer and the adhesive tape. Deionized water containing silicon dust may be introduced into the gap, and the wafer may be contaminated by the silicon dust.
Fourth, since the ultraviolet tape is coated with a vinyl, static electricity occurs when the ultraviolet tape rubs against other objects. The static electricity may damage the semiconductor chips formed on the wafer.